The current trend in the wireless communications industry is towards providing multiple services and worldwide coverage. Due to the co-existing multiple standards and the fact that different services are provided on different frequencies, there is an ever-growing need for multi-band operations and thus the need for multi-band antennas. The rapid development of various radio technologies has dramatically reduced radio volume and thickness. Furthermore, there are emerging technologies, such as time domain radios, which require extremely wide bandwidths, usually well over several hundred megahertz (MHz).
When a radio is operated in either dispatch mode (two-way radio) or phone mode (cellular phones, etc.), antenna efficiency is a major concern. High surface current density antennas, such as wire antennas, restrict currents to small areas. This creates larger near field power densities associated with higher absolute voltages and currents per unit area along the antenna. These types of antennas tend to be susceptible to near field coupling which can result in detuning and reduced far field radiation. Additional circuitry and battery power is often needed to compensate for these losses.
Two alternatives to the wire antenna are the patch antenna and the dispersive surface antenna. FIG. 1 is a front view of a prior art patch antenna structure 100. Antenna structure 100 consists of a radiating element 101 etched on one major surface 102 of a substrate 103. On an opposing substrate surface lies an etched ground plane (not shown). The antenna structure 100 includes an antenna feed 104 for feeding a radio frequency (RF) signal to and from the radiating element 101. Both the radiating element 101 and ground plane are typically made of a low loss conducting material such as copper. Substrate 103 may be made of various materials, such as printed circuit board materials. A disadvantage to the patch antenna is that high field concentrations exist between the radiating element 101 and ground plane. These regions absorb power, which ultimately gets converted to heat loss. Furthermore, most patch antennas have very narrow bandwidths, and those having wider bandwidths generally suffer from higher levels of loss and lower antenna radiation performance. While patch antennas can usually provide a good mechanical fit into most of today's communications devices, they are not, unfortunately, capable of meeting many of the required electrical standards.
FIG. 2 shows a prior art dispersive surface antenna structure 200. Antenna 200 includes a radiating element 201 etched onto one side of a substrate 202 which is located in a plane perpendicular to a ground surface 203, such as a radio case or equivalent. The mounting of antenna structure 200 is similar to that of a common monopole antenna. An RF feed 204 provides an input/output path for current. However, currently available dispersive surface antennas are still unable to provide the flexibility to control the frequency domain characteristics of the antenna.
Accordingly, there is a need for an improved dispersive surface antenna structure that overcomes the problems associated with currently available dispersive surface antennas. An antenna structure providing low surface current density features is highly desirable.